A light emitting device such as a light emitting diode (LED) has been put into practical use for various display boards, a light source for reading an image, a traffic light, a unit for a large size display, a backlight of a cellular phone, and so forth in recent years. The light emitting devices are generally encapsulated with a hardening resin obtained by hardening an aromatic epoxy resin with alicyclic acid anhydride being a hardening agent. However, according to the aromatic epoxy resin base, the alicyclic acid anhydride is easily discolored with an acid, or a long period of time is needed until the resin base is hardened, which is known as a problem. Moreover, when the light emitting device is left outdoors or exposed to a light source emitting ultraviolet light, the resin base has a problem of yellowing of the hardening resin used for encapsulation.
In order to solve such a problem, an attempt has been made to apply a method for encapsulating the LED or the like with a hardening resin using an alicyclic epoxy resin or an acrylic resin, and a cationic polymerization initiator (see Patent literature Nos. 1 and 2). However, the hardening resin subjected to cationic polymerization as described above is very brittle and has a disadvantage of easily developing crack destruction by a cold & hot cycle test (also referred to as a heat cycle test). The hardening resin has also a disadvantage of significant coloring of the hardening resin used for encapsulation after hardening thereof, as compared with the hardening resin that has been applied so far in which the aromatic epoxy resin and the acid anhydride are used. Therefore, the hardening resin is unsuitable for an application in which colorless transparency is required, in particular, an application for encapsulating the LED, in which heat resistance and transparency are required.
Consequently, an examination has been made for a resin composition for an LED encapsulation material, in which development of crack destruction by the cold & hot cycle test is improved and light resistance is excellent (see Patent literature No. 3). The resin composition disclosed herein has a hydrogenated epoxy resin or an alicyclic epoxy resin as a matrix component. However, coloring after hardening thereof is still significant. Thus, a further improvement of resistance to discoloration is desired.
Meanwhile, a white LED is increasingly used in an application for lighting or the like, and disregarding of heat generation of an LED package becomes quite difficult with achieving large power thereof. Because yellowing by the heat generation is no longer avoidable when the epoxy resin is used for an encapsulation material, a silicone resin has been used for the encapsulation material of the white LED in place of the epoxy resin. The silicone resin used for the LED is roughly classified into two types, namely, a phenyl silicone resin and a methyl silicone resin.
The phenyl silicone resin generally used has a satisfactory refractive index value, and is superior in heat resistance to the epoxy resin. However, the phenyl silicone resin is not enough to respond to achieving high power of LED. While the methyl silicone resin is superior in heat resistance and light resistance, the refractive index is low. Therefore, light extraction efficiency of the LED is poor.
Therefore, an encapsulation material that can respond to achieving high power of the white LED, and has both a high refractive index and a good heat resistance to satisfy close contact also, and a thermosetting resin composition used therefor have been required.
Moreover, as compared with the epoxy resin, a hardened material using the silicone resin such as the methyl silicone resin and the phenyl silicone resin has a poorer close contact with a polyamide resin used for an LED substrate and silver used for an electrode, and has had a disadvantage of easily causing peeling by a heat shock or the like.
The methyl silicone resin and the phenyl silicone resin mainly have a polysilsesquioxane compound having a branched structure by a hydrolytic condensation reaction of an alkoxysilane monomer, as a main structure. However, the resins each have a residual silanol group, and therefore have had a problem of causing a change in physical properties, such as a temporal change in hardness by the heat shock or the like. For example, when exposed to high temperature conditions as in a reflow process, the resins have had a disadvantage of easily causing cracks by a hardness rise.
On the other hand, Patent literatures Nos. 4 to 8 disclose a cage-type silicon compound and a polymer thereof, and describe to the effect that heat resistance is satisfactory. The compound and the polymer include a cage-type silsesquioxane in which a structure is controlled, as commonly referred to as a double decker, which is different from a structure of polysilsesquioxane having a random structure ordinarily obtained from the hydrolytic condensation reaction of alkoxysilane. Furthermore, the compound and the polymer do not have a silanol group having a defect in storage stability or causing a hardness rise due to secondary hardening after thermal hardening. Therefore, the compound and the polymer are considered to be suitable for an application such as an LED-use encapsulation material in which long-term reliability is required. However, the compound and the polymer each are solid or crystal, and require a solvent for molding for practical use purpose. Thus, the compound and the polymer as are have been quite difficult to respond to the LED or the like.
Moreover, Patent literature No. 9 discloses an encapsulant-use composition containing a cage-type silicon compound, and an encapsulant. Patent literature 9 discloses a hardened material prepared by obtaining a thermosetting polymer by a hydrosilylation reaction between a cage-type silicon compound having a SiH group and a compound having a vinyl group, and further allowing hardening by hydrosilylation with a compound having a vinyl group. However, the description in Patent literature No. 9 is unclear, and also the hardened material is quite difficult to obtain by the method described in the Patent literature.